1. Field of the Invention
The present invention relates to a Super-Twisted Nematic Liquid Crystal Display (STN-LCD), and more particularly, to a Multi-Line Selection (MLS) driving method for a Super-Twisted Nematic liquid crystal display (STN-LCD).
2. Discussion of Related Art
Due to the low response rate of liquid crystal, it can be difficult to implement a moving picture on a Super-Twisted Nematic Liquid Crystal Display (STN-LCD). For this reason, a liquid crystal with a high response rate, e.g., below 50 msec, has been actively researched and the response rate of liquid crystal has been substantially improved. Accordingly moving images of a certain quality, e.g., frame rate,—level can be implemented in STN-LCD using such an improved liquid crystal.
The Alt-Pleshko Technique (APT) driving method is an example of a method for driving the STN-LCD. One disadvantage of the APT driving method is a “Frame Response”, in which voltages applied to pixels become gradually smaller due to a leakage current, etc. The frame response induces flickers, and the flickers decrease picture quality. To solve this problem, a Multi-Line Selection (MLS) driving method for driving the STN-LCD was developed for displaying high resolution moving images.
According to the MLS driving method, which simultaneously drives a plurality of rows, it is possible to reduce the flickers generated due to the “Frame Response”. In the MLS method, orthogonal row signals are simultaneously applied to selected row lines, and a period of time taken when image signals are applied to the pixels is longer than in the APT method. As the number of simultaneously selected lines increases, picture quality improves, however the driving circuit thereof is more complicated.
Considering the trade-off between the improvement of picture quality and the complexity of driving circuit, a Four Row Line Simultaneous Selection driving method was developed. The four row line simultaneous selection driving method is a method that simultaneously selects and drives four row electrodes. This driving method applies orthogonal row signals to the row lines, that is, alternately applying a positive voltage (+VR) and a negative voltage (−VR) to frames to reduce crosstalk by not applying a DC voltage to the liquid crystal.
FIG. 1 shows the waveforms of a row signal and a column signal for describing the multi-line selection (MLS) driving method used to drive a typical STN-LCD.
With reference to FIG. 1, the multi-line selection (MLS) driving method is a method for applying pixel voltages to liquid crystal, by applying pre-determined column signals (V1–V5) to respective column lines when orthogonal row signals are applied to alternate row lines of a set of four row lines. Here, the respective row lines are selected many times in one frame. Upon the application of a column signal, a plurality of pixel voltages are generated as the difference between the row signal voltage and column signal voltage. A picture image is implemented by turning on/off liquid crystal cells according to root mean square (RMS) values of the varied pixel voltages. Since the RMS values of the pixel voltages determine the on/off states of the liquid crystal cells, levels of the column signal voltages to be applied to the column lines are pre-determined and simultaneously input to the column lines while the row signals are applied to the respective row lines. Combinations of the column signal voltages (V1–V5) and the row signal voltages are created, and RMS values for one frame obtained according to the combinations determine the on/off states of the liquid crystal cells therein.
However, a problem exists in that the conventional MLS driving method used to drive an STN-LCD needs five voltage levels in a column driving circuit, which increases power consumption and makes the driving circuit thereof complicated, in the case where four row lines are simultaneously selected.